Aircraft landing gears include metal components, such as inner and outer cylinders, axles, pins, and actuators, formed from high strength structural materials like steel alloys. The metal components of aircraft landing gears, as well as the metal segments or components of exhaust augmentor flaps or turkey feathers and compressor blades found in aircraft engines, are often encapsulated or at least partially covered by a coating to provide a beneficial effect, such as corrosion resistance. For example, a coating, such as WCCo CrC/NiCr, or (WC/CoCr), may be applied to these metal components by, for example, a plasma spraying technique or by a high velocity oxygen fuel (HVOF) thermal spray process. The coatings are also used to coat shafts, gear boxes, and other wear parts in other industries, such as the oil industry.
Landing gears and exhaust augmentor flaps, as well as compressor blades and other rotating engine parts, are periodically removed from aircraft to inspect the material forming the metal components for stress corrosion, cracking, or other evidence of a condition that could lead to a field failure while in service. The inspection requires that the coating be stripped so that the coating does not interfere with the inspection process. For example, a porous coating may restrict the ability of a penetrant to reach the underlying structural material. If the component passes inspection, a new coating is applied to the metal component before the landing gear or exhaust augmentor flap is returned to service.
Conventional processes for stripping coatings suffer from various deficiencies. For example, a reverse plating process using an aqueous bath containing tartaric acid may be used to remove tungsten-containing coatings. However, this particular reverse plating process is expensive and slow. Aqueous baths containing acids may cause metal components formed from high strength steel alloys to be susceptible to hydrogen ion embrittlement. Embrittled metal components may become susceptible to damage from shock. Although residual trapped hydrogen in the stripped metal components may be removed by a low temperature bake, the required length of the bake slows process throughput.
Stripping solutions are known for removing compounds of titanium from base metals. Generally, these stripping methods immerse the component in a solution including a source of active oxygen and a suitable acid. Under certain conditions, stripping solutions in which hydrogen peroxide acts as the active oxygen source in the composition may spontaneously lose the active oxygen at a dramatic rate. To alleviate this problem, hydrogen peroxide may be replaced with a different type of active oxygen source, such as a chemical substance like sodium perborate tetrahydrate that dissociates in water to form active oxygen. The solution must also include an alkaline source of hydroxyl ions, such as ammonium hydroxide, sodium hydroxide, or potassium hydroxide, which maintains the solution pH at a pH value exceeding 8.0.
Nevertheless, there is a need for improved apparatus, methods, and compositions to efficiently remove coatings from metal components used in aircraft and other aerospace vehicles.